This invention relates to a process of preparing ethylbenzene or substituted derivatives thereof using ethane and benzene, or ethane and substituted benzenes, as raw materials.
Ethylbenzene and substituted ethylbenzenes are useful for preparing styrene and substituted styrenes, which are starting materials for polystyrene plastics.
Ethylbenzene is almost exclusively prepared by the alkylation of benzene with ethylene. Commercial processes include liquid phase alkylation with Friedel-Crafts catalysts, such as aluminum chloride and phosphoric acid, as well as, vapor phase alkylation with acidic carrier catalysts, such as crystalline alumino-silicates. Disadvantageously, these commercial processes require an essentially pure stream of ethylene.
Ethylene is obtained predominantly from the thermal cracking of saturated hydrocarbons, such as natural gas rich in ethane, propane and n- and iso-butane. Alternatively, ethylene can be obtained from the thermal or steam cracking of naphtha. Disadvantageously, crackers also produce a variety of other products, including diolefins and acetylene, which are costly to separate from ethylene. Separation methods include extractive distillation and/or selective hydrogenation of the acetylene back to ethylene. Cracking and separation technologies for the obtention of pure ethylene account for approximately one-third of the total ethylbenzene production costs.
U.S. Pat. No. 5,138,113, for example, teaches a two-step cracking-alkylation process for producing alkylaromatic hydrocarbons from natural gas, the process comprising: 1) thermal cracking of the natural gas with formation of hydrogen and C.sub.2-3 hydrocarbons, particularly, ethylene and acetylene, 2) separation of the C.sub.2-3 hydrocarbons particularly of the ethylene and the acetylene obtained at the end of stage (1) by cooled adsorption in a solvent, and 3) conversion of the C.sub.2-3 hydrocarbons from stage (2) into alkylaromatics. As noted hereinbefore, this type of process is hampered by its dependency on a conventional thermal cracker and cryogenic separation of ethylene from acetylene.
U.S. Pat. No. 4,524,230 discloses an alternative method for preparing alkylaromatic compounds comprising a one-step cracking-alkylation process utilizing a paraffinic hydrocarbon as a source of an alkylating agent. The reaction comprises cracking a paraffinic hydrocarbon into olefinic products on the surface of a non-acid catalyst in the presence of an aromatic compound. The non-acid catalyst comprises a metal from Group VIII of the Periodic Table and may be supported on an alkylation catalyst, preferably, an aluminosilicate zeolite. The olefinic products produced in situ act as alkylating agents towards the aromatic compounds to produce alkylaromatic compounds. Disadvantageously, the selectivity to monoalkylated aromatics is low and unwanted polyalkylated benzenes are produced in large amounts. Moreover, the cracking of the paraffinic hydrocarbon in situ leads to impurities, such as acetylene and diolefins, which reduce the lifetime of the alkylation catalyst.
One method of reducing ethylene costs is to alkylate with dilute ethylene streams, which are available from most refinery FCC units. U.S. Pat. No. 4,107,224 discloses the vapor phase alkylation of benzene to ethylbenzene using a ZSM-5 zeolite as the catalyst. It is taught that the catalyst can handle feedstreams containing from about 15 to about 20 weight percent ethylene while yielding an ethylbenzene purity of greater than 97 weight percent. Disadvantageously, the procurement of the dilute ethylene stream is dependent upon a refinery by-product stream. More disadvantageously, the dilute ethylene stream may contain impurities, such as diolefins and acetylene, which reduce the lifetime of the alkylation catalyst, unless the impure streams are purified first.
In view of the above, it would be desirable to have a process of preparing ethylbenzene or substituted ethylbenzenes which does not rely on conventional thermal crackers and expensive separation technologies for a source of essentially pure ethylene. It would be even more desirable if the process could employ a dilute source of ethylene which is not dependent upon refinery streams and which is not contaminated with impurities which lower the lifetime of the alkylation catalyst.